1. Field of the Invention
The present invention relates to an improvement of the control of an induction motor by a pulse-width-modulated inverter, relating, in particular, to a control that can be fitted to a vehicle driven by induction motors.
2. Description of the Prior Art
As is commonly known, the output voltage and frequency of a pulse-width-modulated inverter can be controlled and is thus suitable for a speed control for an induction motor.
In electric railways, what is called the induction motor type of vehicle using an induction motor, which is convenient for handling and can be expected to have high adhesion characteristics (the friction force between wheels and rail), is being put to practical use.
The main circuit of the induction motor propulsion vehicle consists of an inverter taking in direct current obtained from a direct-current catenary, or from an alternating-current catenary through a rectifier, and providing the induction motor with an alternating current. It is desirable that this inverter is controlled by pulse width modulation (hereinafter referred to as PWM). A typical control system detects the rotational speed f.sub.n of the induction motor and adds or subtracts a prescribed slip frequency f.sub.s. Addition is made during power operation, while subtraction is made during regeneration. The value f.sub.o (=f.sub.n .+-.f.sub.s) thus obtained becomes an instruction for the output frequency of the inverter. Meanwhile, the current in the induction motor is measured and compared with a current instruction. A constant-current control system adjusting the output voltage of the inverter is constituted so that the deviation of the detected value from the value of the instruction becomes zero.
Signals for adjusting this frequency and voltage are input to a PWM control device and the gates of the switching elements constituting the inverter are controlled.
The PWM control device generates a sine wave (the fundamental-wave pattern) having a frequency corresponding to said frequency instruction f.sub.o and a triangular carrier wave used for modulation having a frequency that is an integral multiple of f.sub.o. The peak value of the sine wave is controlled by said variations of current, and is compared with the triangular wave. The result of the comparison becomes a signal indicating the period wherein the status of said switching elements is to be continued or not.
The multiple by which the frequency of the triangular wave is greater than that of the sine wave is the number of voltage pulses within a half-cycle of the output voltage of the inverter. The width of each pulse is adjusted so that the average value of the output voltage is controlled.
There is an optimum number of pulses contained in a half cycle of the output voltage of the inverter value in connection with the rotationed speed of the induction motor. The number of pulses is changed in a sequence related to the rotationed speed.